The invention relates generally to the field of spacecraft attitude control, and more particularly to an apparatus and method for unloading reaction wheel momentum by the use of an opposing reaction wheel. Spacecraft attitude control is required to maintain proper orientation of the spacecraft related to a planet. Currently spacecraft attitude control may be accomplished by spinning the spacecraft and by mounting the sensors or antennas on a despun platform. Alternatively, the spacecraft may be stabilized in three axes by an attitude control system using fuel-burning thrusters, but the use of such thrusters requires the expenditure of fuel, which tends to limit the service life of the spacecraft. Another method of three-axis attitude control is the use of reaction wheels or momentum wheels to effect precision attitude control. The reaction wheels are positioned within and rotate independently of the spacecraft. This rotation causes a torque to act on the inertial mass of the spacecraft, thereby inducing a radial acceleration that causes attitudinal movement of the spacecraft. In this way the torque created by the reaction wheels is used to effect attitude control. Environmental disturbance torque""s that act on the spacecraft tend to result in a gradual increase in the wheel momenta. In order to prevent the wheels from reaching their mechanical momentum limits, and to thereby maintain the ability to provide reaction wheel based attitude control, the wheel momentum must be periodically reduced or desaturated. Wheel momentum may be desaturated by the use of fuel consuming thrusters, which torque the spacecraft body in a manner which opposes the reaction wheel momentum. However, the use of such thrusters is subject to the same objection of limited lifetime described above. Another method of reaction wheel desaturation is the use of magnetic desaturation, which uses magnetic torquers to cause a decrease in the wheel speed. In the simplest form, the torquers are solenoids that are energized to cause current flow therein. When energized, a magnetic field is induced. The resulting magnetic field seeks to align itself with the earth""s magnetic field causing a torque to act on the spacecraft body in a manner, which opposes the reaction wheel momentum. Magnetic torquers can not cause torque in the north south plane, also magnetic torquers are very massive, very slow and use a great amount of electricity. The invention eliminates the need for thruster desaturation like magnetic desaturation but would have less mass and perform desaturation faster and consume less electricity for the same amount of desaturation, which means a reaction wheel could be made with a very small mass but would need to be desaturated often. Another benefit of the invention over magnetic desaturation is that the invention would perform better in high earth orbits or other planets where there is magnetic fields of less magnitude.
The present invention is an attitude control actuation system, which uses two reaction wheels for each of the three axis. The two reaction wheels first rotational axis are initially in the same plane. The two reaction wheels each start off rotating in the same direction to exchange an equal amount of angular momentum with the spacecraft. Then the invention performs desaturation of the two reaction wheels by free spinning both of the reaction wheels about a second axis, which is perpendicular to the first axis, while being rotated about a third axis, which is perpendicular to the second axis. The two reaction wheels would be rotated so that the two reaction wheels bring their own first axis angular momentum in plane with the third axis plane and are then rotating in the same plane but in opposite directions and then can be slowed down by exchanging momentum with each other.